I have a 75 H/O W30

Does anyone know the OEM tires size that came on it?

Does anyone know the fattest tire I can put under it without ruppbin on turns?

It currently has 235/70r15's on 15x7 stock rims.

 

I'm pretty sure the originals were Goodyear Polyglas H70-15s.
Modern equivalents would be the 235/70/15s you have.

'73-'77s have rather large wheel wells - I'd bet 255/60/15 - H60-15 (or 245/60/15 - G60-15) front & 275/60/15 - L60-15 rears on your original 15x7.0" SSIIIs would work well - but I don't have first hand experience.

 

Check here:

http://www.73-77olds.com/forums/index.php

for more help.

 

Courtesy of Wild About Cars:

Section 10 of the Assembly Manual Covers tires.
http://wildaboutcarsonline.com/cgi-b...aldisplayed=50

And there is this 1975 Hurst/Olds Announcement and Specifications:
http://wildaboutcarsonline.com/cgi-b...aldisplayed=50

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